If you receive a text message on your iPhone any time after Thursday afternoon containing only a single square character, Charlie Miller would suggest you turn the device off. Quickly.

That small cipher will likely be your only warning that someone has taken advantage of a bug that Miller and his fellow cybersecurity researcher Collin Mulliner plan to publicize Thursday at the Black Hat cybersecurity conference in Las Vegas. Using a flaw they’ve found in the iPhone’s handling of text messages, the researchers say they’ll demonstrate how to send a series of mostly invisible SMS bursts that can give a hacker complete power over any of the smart phone’s functions. That includes dialing the phone, visiting Web sites, turning on the device’s camera and microphone and, most importantly, sending more text messages to further propagate a mass-gadget hijacking.

“This is serious. The only thing you can do to prevent it is turn off your phone,” Miller told Forbes. “Someone could pretty quickly take over every iPhone in the world with this.”

Though Miller and Mulliner say they notified Apple about the vulnerability more than a month ago, the company hasn’t released a patch, and it didn’t respond to Forbes’ repeated calls seeking comment.

The iPhone SMS bug is just one of a series that the researchers plan to reveal in their talk. They say they’ve also found a similar texting bug in Windows Mobile that allows complete remote control of Microsoft-based devices. Another pair of SMS bugs in the iPhone and Google’s Android phones would purportedly allow a hacker to knock a phone off its wireless network for about 10 seconds with a series of text messages. The trick could be repeated again and again to keep the user offline, Miller says. Though Google has patched the Android flaw, this second iPhone bug also remains unpatched, he adds.

The new round of bugs aren’t the first that Miller has dug up in the iPhone’s code. In 2007, he became the first to remotely hijack the iPhone using a flaw in its browser. But while that vulnerability gave the attacker a similar power over the phone’s functions, it required tricking the user into visiting an infected Web site to invisibly download a piece of malicious software. When Miller alerted Apple in July of that year, the company patched the vulnerability before Miller publicized the bug at the Black Hat conference the following month.